We have previously shown that a wide variety of virally and spontaneously transformed rodent fibroblasts secrete a major transformation-dependent phosphoprotein with a molecular weight of about 62 kilodaltons. Elevated secretion of the phosphoprotein occurs regardless of the transforming agent, and secretion is temperature sensitive in a cell line that is temperature sensitive for the transformed phenotype. Elevated secretion of this major phosphoprotein also correlates with tumorigenicity of cells of epithelial origin and distinguishes neoplastic from hyperplastic epithelium. The 62 kilodalton phosphoprotein is apparently not related to viral or tumor-specific phosphoproteins that have been described by others. Within the past year we have determined that a variety of tumorigenic human cell lines (i.e., tumorigenic in the nude mouse) also secrete this phosphoprotein while non-tumorigenic human cell lines do not. Hence our earlier observations with rodent cell lines extend to human cells as well. The tumorigenic human cells that express the phosphoprotein were derived from such diverse tumors as colonic carcinoma, fibrosarcoma, osteogenic sarcoma, and ovarian teratocarcinoma. We have also determined that both normal rat serum and normal human serum contain proteins that share antigenic determinants with the 62 kilodalton molecular weight, transformation-dependent phosphoprotein. We are in the process of determining if these normal serum proteins are related to any of the serum proteins that have previously been described. (B)